1. Field of the Invention
The present invention relates to a method and an apparatus for transmitting/receiving Channel State Information (CSI) in a Coordinated Multi-Point (CoMP) communication system.
2. Description of the Related Art
A communication system has uplink and downlink channels. The downlink channels are established for transmission from at least one Transmission Point (TP) to User Equipments (UEs). The uplink channels are established for transmission from the UEs to at least one Reception Point (RP). The UE is typically referred to as a terminal or a mobile station. The UE can be a fixed UE or a mobile UE. The UE can be any one of a radio device, a cellular phone, a personal computer device, and the like. The TP or RP is typically a fixed station. The TP and RP can be integrated into a single device which can be referred to as base station. The base station can be referred to as any one of a Base Transceiver System (BTS), a Node B, an enhanced Node B (eNB), an Access Point (AP), and the like.
The communication system supports transmission of diverse signals including a data signal, a control signal, and a reference signal. The data signal carries information content. The control signal is capable of processing the data signal appropriately. The reference signal can also be referred to as a pilot signal. The reference signal is capable of coherent demodulation on the data and control signals. The reference signal makes it possible to generate the Channel State Information (CSI) corresponding to the estimation value of the channel medium.
The Uplink (UL) data is carried by a Physical Uplink Shared Channel (PUSCH). The UE transmits a PUSCH, and the UL Control Information (UCI) is transmitted through Physical Uplink Control Information (PUCCH) with the exception of the case where the UE is capable of transmitting the data and a part of UCI through the PUSCH. The UCI includes acknowledgement (ACK) information in association with a Hybrid Automatic Repeat Request (HARQ) process. The HARQ-ACK is of acknowledging the receipt of the Transport Blocks (TB) transmitted to the UE in downlink of the communication system and corresponds to the signal transmitted from the Node B to the UE.
The DL TBs are transmitted on a Physical Downlink Shared Channel (PDSCH). The UCI may include at least one of a Channel Quality Indicator (CQI), a Precoding Matrix Indicator (PMI), and a Rank Indicator (RI). The CQI, PMI, and RI can be integrally referred to as Channel State Information (CSI). The CQI provides the Node B with the Signal to Interference and Noise Ratio (SINR) which the UE experiences across sub-bands or entire DL operating bandwidth (BW). Typically, the measurement value is provided in the form of the best Modulation and Coding Scheme (MCS) level at which a predefined BLock Error Rate (BLER) is accomplished. The Node B can be notified of the method of combining the signals transmitted from the UE to the Node B antennas in a Multiple-Input Multiple-Output (MIMO) scheme with PMI/RI. The UE is capable of transmitting the UCI on PUCCH in separation from data or PUSCH along with data.
The DL data is transmitted on PDSCH. The DL Control Information (DCI) includes at least one of a DL CSI feedback request, a UL Scheduling Assignments for PUSCH transmission (UL SAs), and a DL Scheduling Assignments for PDSCH reception (DL SAs). The SAs are notified by the DCI formats transmitted on a Physical Downlink Control Channel (PDCCH). In addition to SAs, the PDCCHs may carry a common DCI for all UEs or a group of UEs.
FIG. 1 is a graph illustrating a resource for use in a Long Term Evolution-Advanced (LTE-A) system according to the related art.
Referring to FIG. 1, in the LTE and LTE-A, the DL transmission is performed in a unit of a time-domain subframe and a frequency-domain Resource Block (RB). A subframe spans 1 msec, and an RB consists of 12 subcarriers corresponding to the transmission bandwidth of 180 kHz. As shown in FIG. 1, the system bandwidth of LTE-A is divided into plural RBs in the frequency domain and plural subframes in the time domain.
The LTE-A Release 10 and beyond systems may operate with different signals. In downlink, the following reference signals are transmitted.
1. A Cell Specific Reference Signal (CRS): Used in an initial system access, paging, a PDSCH demodulation, a channel measurement, a handover, and the like.
2. A Demodulation Reference Signal (DMRS): used for demodulation of a PDSCH.
3. A Channel Status Information Reference Signal (CSI-RS): Used for channel measurement.
In addition to these reference signals, zero-power CSI-RS can be adopted to the LTE-A release 10. Although the zero power CSI-RS may occur at the same time and frequency resources as the normal CSI-RS, it differs from the normal CSI-RS in that the REs to which the zero power CSI-RS are mapped have no transmission. The zero power CSI-RS aims at muting CSI-RS transmission of a specific TP on the resource used by the adjacent TPs so as to avoid interference to the CSI-RSs transmitted by the adjacent TPs.
FIG. 2 is a diagram illustrating an RB for use in an LTE/LTE-A system according to the related art.
Referring to FIG. 2, the RB consists of Resource Elements (REs) to which diverse reference signals, PDSCH, zero power CSI-RS and control channels are mapped. It is noted that FIG. 2 shows a single RB in the frequency domain and a single subframe in the time domain. A subframe may include a plurality of RBs that can be used for transmitting the aforementioned signals. In FIG. 2, the resources marked with A, B, C, D, E, F, G, H, I, and J correspond to 4 CSI-RS ports. For example, the 4 REs marked with ‘A’ are used for CSI-RS transmission with 4 antenna ports. The CSI-RS of 2 antenna ports can be transmitted on the resource acquired by restricting the resource for CSI-RS of 4 antenna ports to 2. Additionally, the CSI-RS of 8 antenna ports can be transmitted on the resource acquired by combining the two resources for CSI-RS of the 4 antenna ports. The zero power CSI-RS can be mapped to the resources for CSI-RS of 4 antenna ports.
In the DL transmission mode 9 of the 3rd Generation Partnership Project (3GPP) LTE-A release 10, the UEs measures the CSI-RS transmitted by the eNB and feeds back downlink Channel Status Information (CSI), such as RI, PMI, and CQI. The RI, PMI, and CQI are reported at the respective timings indicated by the eNB. In a CSI feedback, PMI is calculated based on the most recently reported RI while CQI is calculated under the assumption of the most recently reported RI and PMI.
Meanwhile, one of the key issues in the communication systems is the enhancement of a cell area and system throughput. The Coordinated Multi-Point (CoMP) transmission/reception is one of the significant techniques to accomplish these aims. The CoMP relies on the fact that the UE located at the cell edge is capable of receiving the downlink signal transmitted via a set of TPs more reliability (DL CoMP) and transmitting the uplink signal via a set of RPs more reliably (UL CoMP). The DL CoMP may include the relatively simple interference avoidance methods, such as a coordinated scheduling and complex methods requiring accurate and detailed channel information, such as a coordinated transmission of plural TPs. The UL CoMP may include the simple methods, such as a PUSCH scheduling in consideration of a single RP and more complex methods based on the received signal characteristics from multiple RPs and in consideration of interference.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.